Water splitting refers to the chemical reaction of water to its constituent elements in the form of diatomic hydrogen and diatomic oxygen. Photoelectrochemical (PEC) water splitting is a promising pathway to the economical production of solar hydrogen. Using a semiconductor to directly split water eliminates the high capital costs of an electrolyzer. The key parameters that dictate hydrogen costs from the PEC approach are semiconductor efficiency, stability, and cost. Technoeconomic analysis for PEC hydrogen production reveals that high solar-to-hydrogen (STH) efficiency is the most critical figure of merit when designing such a system.
Over the last four decades, researchers have evaluated transition metal oxide semiconductors for PEC water splitting. However, wide bandgaps and poor optoelectronic properties limit their efficiencies to a few percent, precluding them from approaching the efficiencies necessary for economical large-scale manufacturing and use. Hybrid photoelectrodes, with a transition metal oxide photoanode mechanically stacked on a photovoltaic (PV) device, have recently achieved 3.1% and 4.9% solar-to-hydrogen conversion efficiencies, which are close to the upper limits accessible based on the properties of the WO3 and BiVO4 anodes incorporated by these systems.
Dual PEC junction III-V-based systems, constructed with p-InP coupled to an n-GaAs photoanode, have achieved STH efficiencies up to 8%. A related art is the GaInP2/GaAs tandem cell, which achieved up to about 12.4% STH at short-circuit and up to about 16% STH with a small (240 mV) bias (see Khaselev, O.; Turner, J.; Science 1998, 280, 425-7 and Turner, J. A.; Deutsch, T. G., http://www.hydrogen.energy.gov/pdfs/review11/pd035_turner_2011_o.pdf). However, despite these commendable successes, significant improvements are still needed before PEC technology for water-splitting can be considered a viable and economical method for hydrogen fuel production.